patch 9.0.0970: Coverity warns for uninitialized variable

Problem:    Coverity warns for uninitialized variable.
Solution:   Initialize "ren_ret".
diff --git a/src/if_py_both.h b/src/if_py_both.h
index 0cde18d..5589af6 100644
--- a/src/if_py_both.h
+++ b/src/if_py_both.h
@@ -5274,7 +5274,7 @@
     {
 	char_u		*val;
 	aco_save_T	aco;
-	int		ren_ret;
+	int		ren_ret = OK;
 	PyObject	*todecref;
 
 	if (!(val = StringToChars(valObject, &todecref)))